Method for making light emitting diode

ABSTRACT

A method for making a light emitting diode, the method includes the following steps. First, a substrate having an epitaxial growth surface is provided. Second, a carbon nanotube layer is placed on the epitaxial growth surface. Third, a first semiconductor layer, an active layer and a second semiconductor layer are grown on the epitaxial growth surface. Fourth, a portion of the second semiconductor layer and the active layer is etched to expose a portion of the first semiconductor layer. Fifth, a first electrode is prepared on the first semiconductor layer and a second electrode is prepared on the second semiconductor layer. Sixth, the carbon nanotube layer is removed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims all benefits accruing under 35 U.S.C. §119 fromChina Patent Application No. 201110110759.1, filed on Apr. 29, 2011, inthe China Intellectual Property Office, the contents of which are herebyincorporated by reference. This application is related tocommonly-assigned applications entitled “METHOD FOR MAKING LIGHTEMITTING DIODE”, filed on Nov. 3, 2011, Ser. No. 13/288,174; “LIGHTEMITTING DIODE”, filed on Nov. 3, 2011, Ser. No. 13/288,180; “METHOD FORMAKING LIGHT EMITTING DIODE”, filed on Nov. 3, 2011, Ser. No.13/288,183; “LIGHT EMITTING DIODE”, filed on Nov. 3, 2011, Ser. No.13/288,187; “METHOD FOR MAKING LIGHT EMITTING DIODE”, filed on Nov. 3,2011, Ser. No. 13/288,192; “LIGHT EMITTING DIODE”, filed on Nov. 3,2011, Ser. No. 13/288,327; “LIGHT EMITTING DIODE”, filed on Nov. 3,2011, Ser. No. 13/288,203; “METHOD FOR MAKING LIGHT EMITTING DIODE”,filed on Nov. 3, 2011, Ser. No. 13/288,213; “LIGHT EMITTING DIODE”,filed on Nov. 3, 2011, Ser. No. 13/288,222; “LIGHT EMITTING DIODE”,filed on Nov. 3, 2011, Ser. No. 13/288,234; “LIGHT EMITTING DIODE”,filed on Nov. 3, 2011, Ser. No. 13/288,238. The disclosures of theabove-identified applications are incorporated herein by reference.

BACKGROUND

1. Technical Field

The present disclosure relates to a light emitting diode (LED) and amethod for making the same.

2. Description of Related Art

In recent years, highly efficient LEDs made with GaN-basedsemiconductors have been widely applied in different technologies, forexample in display devices, large electronic billboards, street lights,car lights, and other illumination applications. LEDs areenvironmentally friendly, and have long working life, and low powerconsumption.

A conventional LED commonly includes an N-type semiconductor layer, aP-type semiconductor layer, an active layer, an N-type electrode and aP-type electrode. The active layer is located between the N-typesemiconductor layer and the P-type semiconductor layer. The P-typeelectrode is located on the P-type semiconductor layer. The N-typeelectrode is located on the N-type semiconductor layer. Typically, theP-type electrode is transparent. In operation, a positive voltage and anegative voltage are applied respectively to the P-type semiconductorlayer and the N-type semiconductor layer. Thus, holes in the P-typesemiconductor layer and electrons in the N-type semiconductor layer canenter the active layer and combine with each other to emit visiblelight.

However, extraction efficiency of LEDs is low because typicalsemiconductor materials have a higher refractive index than that of air.Large-angle light is light traveling at an angle defined between thelight and a medium boundary. If the angle is larger than a particularcritical angle, the light will be internally reflected. Large-anglelight emitted from the active layer may be internally reflected in LEDs,so that a large portion of the light emitted from the active layer willremain in the LEDs, thereby degrading the extraction efficiency.

What is needed, therefore, is a LED which can overcome theabove-described shortcomings.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the embodiments can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily drawn to scale, the emphasis instead being placed uponclearly illustrating the principles of the embodiments. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 is a flowchart of one embodiment of a method for making an LED.

FIG. 2 is a Scanning Electron Microscope (SEM) image of a drawn carbonnanotube film used in the method of FIG. 1.

FIG. 3 is a schematic structural view of a carbon nanotube segment ofthe drawn carbon nanotube film of FIG. 2.

FIG. 4 is an SEM image of cross-stacked carbon nanotube films used inthe method of FIG. 1.

FIG. 5 is an SEM image of untwisted carbon nanotube wire used in themethod of FIG. 1.

FIG. 6 is an SEM image of twisted carbon nanotube films used in themethod of FIG. 1.

FIG. 7 is a transmission electron microscopy (TEM) of a cross-sectionalview of the first semiconductor layer and the substrate of the LED madeby the method of FIG. 1.

FIG. 8 is a schematic structural view of an LED made by the method ofFIG. 1.

FIG. 9 is a flowchart of one embodiment of a method for making an LED.

FIG. 10 is a schematic structural view of an LED made by the method ofFIG. 9.

DETAILED DESCRIPTION

The disclosure is illustrated by way of example and not by way oflimitation in the figures of the accompanying drawings in which likereferences indicate similar elements. It should be noted that referencesto “an” or “one” embodiment in this disclosure are not necessarily tothe same embodiment, and such references mean at least one.

Referring to FIG. 1, a method for making an LED of one embodimentincludes the following steps:

S10, providing a substrate 102 having an epitaxial growth surface 122;

S20, placing a carbon nanotube layer 104 on the epitaxial growth surface122;

S30, growing a first semiconductor layer 106, an active layer 108, and asecond semiconductor layer 110 on the epitaxial growth surface 122;

S40, exposing a portion of the first semiconductor layer 106 by etchinga portion of the second semiconductor layer 110 and the active layer108;

S50, preparing a first electrode 114 on the first semiconductor layer106, and preparing a second electrode 112 on the second semiconductorlayer 110; and

S60, removing the carbon nanotube layer 104.

In step S10, the epitaxial growth surface 122 can be used to grow thefirst semiconductor layer 106. The epitaxial growth surface 122 is aclean and smooth surface. The substrate 102 can be made of transparentmaterial. The substrate 102 is used to support the first semiconductorlayer 106. The substrate 102 can be a single-layer structure or amulti-layered structure. If the substrate 102 is a single-layerstructure, the substrate 102 can be a single crystal structure having acrystal face. The crystal face can be used as the epitaxial growthsurface 122. If the substrate 102 is the signal-crystal structure, thematerial of the substrate 102 can be made of SOI (silicon on insulator),LiGaO₂, LiAlO₂, Al₂O₃, Si, GaAs, GaN, GaSb, InN, InP, InAs, InSb, AlP,AlAs, AlSb, AlN, SiC, SiGe, GaMnAs, GaAlAs, GaInAs, GaAlN, GaInN, AlInN,GaAsP, InGaN, AlGaInN, AlGaInP, Aperture:Zn or Aperture:N. If thesubstrate 102 is a multi-layer structure, the substrate 102 shouldinclude at least one layer of the above-described single crystalstructure having a crystal face. The size, thickness and shape of thesubstrate 102 can be selected according to need. In one embodiment, thesubstrate 102 is made of sapphire.

In step S20, the carbon nanotube layer 104 includes a number of carbonnanotubes. A thickness of the carbon nanotube layer 104 can be in arange from 1 nm to 100 μm, for example about, 1 nm, 10 nm, 200 nm, 1 μmor 10 μm. In one embodiment, the thickness of the carbon nanotube layer104 is about 100 nm. The length and diameter of the carbon nanotubes inthe carbon nanotube layer 104 are selected according to need. The carbonnanotubes in the carbon nanotube layer 104 can be single-walled,double-walled, multi-walled carbon nanotubes, or combinations thereof.

The carbon nanotube layer 104 forms a pattern so part of the epitaxialgrowth surface 122 can be exposed from the patterned carbon nanotubelayer 104 after the carbon nanotube layer 104 is placed on the epitaxialgrowth surface 122. Thus, the first semiconductor layer 106 can growfrom the exposed epitaxial growth surface 122.

The patterned carbon nanotube layer 104 defines a number of apertures105. The apertures 105 are dispersed uniformly. The apertures 105 extendthrough the carbon nanotube layer 104 along a thickness direction of thecarbon nanotube layer 104. Therefore, the carbon nanotube layer 104 is agraphical structure. The carbon nanotube layer 104 covers the epitaxialgrowth surface 122 of the substrate 102. A portion of the epitaxialgrowth surface 122 is then exposed from the apertures 105 of the carbonnanotube layer 104, and the first semiconductor layer 106 grows from theapertures 105 of the carbon nanotube layer 104. The aperture 105 can bea hole defined by several adjacent carbon nanotubes or a gap defined bytwo substantially parallel carbon nanotubes and extending along axialdirections of the carbon nanotubes. The size of the apertures 105 can bethe diameter of the hole or width of the gap, and can be in a range fromabout 10 nanometers to about 500 micrometers. The hole-shaped apertures105 and the gap-shaped apertures 105 can exist in the patterned carbonnanotube layer 104 at the same time. The sizes of the apertures 105within can be different. The sizes of the apertures 105 can be, forexample about, 1 nanometer, 10 nanometers, 50 nanometers, 80 nanometers,100 nanometers, 120 nanometers, or 500 nanometers. The smaller the sizesof the apertures 105, the less dislocation defects will occur during theprocess of growing the first semiconductor layer 106. In one embodiment,the sizes of the apertures 105 are in a range from about 10 nanometersto about 10 micrometers. The duty factor of the carbon nanotube layer104 is an area ratio between the sheltered epitaxial growth surface 122and the exposed epitaxial growth surface 122. The dutyfactor of thecarbon nanotube layer 104 can be in a range from about 1:100 to about100:1, such as about, 1:10, 1:2, 1:4, 4:1, 2:1 or 10:1. In oneembodiment, the dutyfactor of the carbon nanotube layer 104 is in arange from about 1:4 to about 4:1.

In one embodiment, the carbon nanotubes in the carbon nanotube layer 104are arranged to extend along the direction substantially parallel to thesurface of the carbon nanotube layer 104 to obtain a pattern and greaterlight transmission. After being placed on the epitaxial growth surface122, the carbon nanotubes in the carbon nanotube layer 104 are arrangedto extend along the direction substantially parallel to the epitaxialgrowth surface 122. Referring to FIG. 2, all the carbon nanotubes in thecarbon nanotube layer 104 are arranged to substantially extend along thesame direction. Referring to FIG. 4, part of the carbon nanotubes in thecarbon nanotube layer 104 are arranged to extend along a firstdirection. The other part of the carbon nanotubes in the carbon nanotubelayer 104 are arranged to extend substantially along a second direction,perpendicular to the second direction. Also the carbon nanotubes in theordered carbon nanotube structure can be arranged to extend along thecrystallographic orientation of the substrate 102 or along a direction,which forms an angle with the crystallographic orientation of thesubstrate 102.

The carbon nanotube layer 104 can be formed on the epitaxial growthsurface 122 by chemical vapor deposition (CVD), transfer printing apreformed carbon nanotube film, or filtering and depositing a carbonnanotube suspension. In one embodiment, the carbon nanotube layer 104 isa free-standing structure and can be drawn from a carbon nanotube array.The term “free-standing structure” means that the carbon nanotube layer104 can sustain the weight of itself if it is hoisted by a portionthereof without any significant damage to its structural integrity.Thus, the carbon nanotube layer 104 can be suspended by two spacedsupports. The free-standing carbon nanotube layer 104 can be laid on theepitaxial growth surface 122 directly and easily.

The carbon nanotube layer 104 can be a continuous structure or adiscontinuous structure. The discontinuous carbon nanotube layer 104includes a number of carbon nanotube wires substantially parallel toeach other. If the carbon nanotube layer 104 has carbon nanotube wiressubstantially parallel to each other and a supporting force is appliedto the carbon nanotube layer 104 in a direction substantiallyperpendicular to axial directions of the carbon nanotube wires, theparallel carbon nanotube wires can form a free-standing structure. Thesuccessive carbon nanotubes are joined end to end by van der Waalsattractive force in a direction substantially parallel to an axialdirection of the carbon nanotube. The carbon nanotubes are connectedwith each other by van der Waals attractive force in a directionsubstantially perpendicular to an axial direction of the carbonnanotubes.

The carbon nanotube layer 104 can be a substantially pure structure ofthe carbon nanotubes, with few impurities and chemical functionalgroups. The carbon nanotube layer 104 can be a composite including acarbon nanotube matrix and some non-carbon nanotube materials. Thenon-carbon nanotube materials can be graphite, graphene, siliconcarbide, boron nitride, silicon nitride, silicon dioxide, diamond,amorphous carbon, metal carbides, metal oxides, or metal nitrides. Thenon-carbon nanotube materials can also be coated on the carbon nanotubesof the carbon nanotube layer 104 or filled in the apertures 105. In oneembodiment, the non-carbon nanotube materials are coated on the carbonnanotubes of the carbon nanotube layer 104 so the carbon nanotubes canhave greater diameters and the apertures 105 can have smaller sizes. Thenon-carbon nanotube materials can be deposited on the carbon nanotubesof the carbon nanotube layer 104 by CVD, physical vapor deposition(PVD), and sputtering.

Furthermore, the carbon nanotube layer 104 can be treated with anorganic solvent after being placed on the epitaxial growth surface 122so the carbon nanotube layer 104 can be attached on the epitaxial growthsurface 122 firmly. Specifically, the organic solvent can be applied tothe entire surface of the carbon nanotube layer 104 or the entire carbonnanotube layer 104 can be immerged in an organic solvent. The organicsolvent can be volatile, for example ethanol, methanol, acetone,dichloroethane, chloroform, or mixtures thereof. In one embodiment, theorganic solvent is ethanol.

The carbon nanotube layer 104 can include at least one carbon nanotubefilm, at least one carbon nanotube wire, or combination thereof. In oneembodiment, the carbon nanotube layer 104 can include a single carbonnanotube film or two or more stacked carbon nanotube films. Thus, thethickness of the carbon nanotube layer 104 can be controlled by thenumber of the stacked carbon nanotube films. The number of the stackedcarbon nanotube films can be in a range from about 2 to about 100, forexample 10, 30, or 50. In one embodiment, the carbon nanotube layer 104can include a layer of parallel and spaced carbon nanotube wires. Also,the carbon nanotube layer 104 can include a plurality of carbon nanotubewires crossed or weaved together to form a carbon nanotube net. Thedistance between two adjacent parallel and spaced carbon nanotube wirescan be in a range from about 0.1 nm to about 200 nm. In one embodiment,the distance between two adjacent parallel and spaced carbon nanotubewires can be in a range from about 10 nm to about 100 nm. The size ofthe apertures 105 can be controlled by the distance between two adjacentparallel and spaced carbon nanotube wires. The length of the gap betweentwo adjacent parallel carbon nanotube wires can be equal to the lengthof the carbon nanotube wire. Any carbon nanotube structure described canbe used with all embodiments.

A drawn carbon nanotube film is composed of a plurality of carbonnanotubes. A large majority of the carbon nanotubes in the drawn carbonnanotube film can be oriented along a preferred orientation, meaningthat a large majority of the carbon nanotubes in the drawn carbonnanotube film are arranged substantially along the same direction. Anend of one carbon nanotube is joined to another end of an adjacentcarbon nanotube arranged substantially along the same direction by vander Waals attractive force. The drawn carbon nanotube film is capable offorming a freestanding structure. The successive carbon nanotubes joinedend to end by van der Waals attractive force realizes the freestandingstructure of the drawn carbon nanotube film.

Some variations can occur in the orientation of the carbon nanotubes inthe drawn carbon nanotube film. Microscopically, the carbon nanotubesoriented substantially along the same direction may not be perfectlyaligned in a straight line, and some curve portions may exist. A contactbetween some carbon nanotubes located substantially side by side andoriented along the same direction cannot be totally excluded.

The structure of the drawn carbon nanotube film and the method formaking the drawn carbon nanotube film is illustrated as follows.

Referring to FIGS. 2 and 3, each drawn carbon nanotube film includes aplurality of successively oriented carbon nanotube segments 143 joinedend-to-end by van der Waals attractive force therebetween. Each drawncarbon nanotube segment 143 includes a plurality of carbon nanotubes 145substantially parallel to each other, and combined by van der Waalsattractive force therebetween. The drawn carbon nanotube segments 143can vary in width, thickness, uniformity, and shape. The carbonnanotubes in the drawn carbon nanotube film are also substantiallyoriented along a preferred orientation. A thickness of the drawn carbonnanotube film can range from about 1 nm to about 100 μm in oneembodiment. The thickness of the drawn carbon nanotube film can rangefrom about 100 nm to about 10 nm in another embodiment. A width of thedrawn carbon nanotube film relates to the carbon nanotube array fromwhich the drawn carbon nanotube film is drawn. The apertures between thecarbon nanotubes in the drawn carbon nanotube film can form theapertures 105 in the carbon nanotube layer 104. The apertures betweenthe carbon nanotubes in the drawn carbon nanotube film can be less than10 nm. Examples of the drawn carbon nanotube film are taught by U.S.Pat. No. 7,045,108 to Jiang et al., and WO 2007015710 to Zhang et al.

The carbon nanotube layer 104 includes at least two drawn carbonnanotube films stacked with each other. In other embodiments, the carbonnanotube layer 104 can include two or more coplanar carbon nanotubefilms, and each coplanar carbon nanotube film can include multiplelayers. Additionally, if the carbon nanotubes in the carbon nanotubefilm are aligned along one preferred orientation (e.g., the drawn carbonnanotube film), an angle can exist between the orientation of carbonnanotubes in adjacent films, whether stacked or adjacent. Adjacentcarbon nanotube films are combined by van der Waals attractive forcetherebetween. An angle between the aligned directions of the carbonnanotubes in the two adjacent drawn carbon nanotube films can range fromabout 0 degrees to about 90 degrees)(0°≦α≦90°). If α=0°, the twoadjacent drawn carbon nanotube films are arranged in the same directionwith each other. If the angle between the aligned directions of thecarbon nanotubes in adjacent stacked drawn carbon nanotube films islarger than 0 degrees, a plurality of micropores is defined by thecarbon nanotube layer 104. Referring to FIG. 6, the carbon nanotubelayer 104 shown with the angle between the aligned directions of thecarbon nanotubes in adjacent stacked drawn carbon nanotube films is 90degrees. The stacked drawn carbon nanotube films can improve thestrength and maintain the shape of the carbon nanotube layer 104.Stacking the carbon nanotube films also increase the structuralintegrity of the carbon nanotube layer 104.

Furthermore, the carbon nanotube layer 104 can be heated to decrease thethickness of the carbon nanotube layer 104. If the carbon nanotube layer104 is heated, the carbon nanotubes with larger diameters will absorbmore energy and be destroyed. The carbon nanotube layer 104 can beheated locally to protect the carbon nanotube layer 104 from damage. Inone embodiment, the carbon nanotube layer 104 is heated by the followingsteps: (1) dividing a surface of the carbon nanotube layer 104 into anumber of local areas; (2) heating all of the local areas of the carbonnanotube layer 104 one by one. The carbon nanotube layer 104 can beheated by a laser or a microwave. In one embodiment, the carbon nanotubelayer 104 is heated by the laser and a power density of the laser isgreater than 0.1×10⁴ W/m².

The laser can irradiate the carbon nanotube layer 104 in many ways. Thedirection of the laser can be substantially perpendicular to the surfaceof the carbon nanotube layer 104. The moving direction of the laser canbe substantially parallel or perpendicular to axial directions of thecarbon nanotubes in the carbon nanotube layer 104. For a laser with astable power density and wavelength, the slower the moving speed of thelaser, the more carbon nanotubes of the carbon nanotube layer 104 willbe destroyed, and the thinner the carbon nanotube layer 104. However, ifthe speed is too slow, the carbon nanotube layer 104 will be completelydestroyed. In the present embodiment, a power density of the laser isabout 0.053×10¹² W/m², a diameter of the irradiating pattern of thelaser is in a range from about 1 mm to about 5 mm, and a laserirradiation time is less than 1.8 seconds. In the present embodiment,the laser is a carbon dioxide laser and the power density of the laseris about 30 W, a wavelength of the laser is about 10.6 μm, a diameter ofthe irradiating pattern of the laser is about 3 mm, and a moving speedof the laser device is less than 10 m/s.

The carbon nanotube wire can be an untwisted carbon nanotube wire or atwisted carbon nanotube wire. Both the untwisted carbon nanotube wire ortwisted carbon nanotube wire can be a free-standing structure. Referringto FIG. 5, the untwisted carbon nanotube wire includes a plurality ofcarbon nanotubes substantially oriented along a direction along thelength of the untwisted carbon nanotube wire. Specifically, theuntwisted carbon nanotube wire includes a plurality of successive carbonnanotube treated segments joined end to end by van der Waals attractiveforce therebetween. Each carbon nanotube treated segment includes aplurality of carbon nanotubes substantially parallel to each other, andcombined by van der Waals attractive force therebetween. The carbonnanotube treated segments can vary in width, thickness, uniformity, andshape. The length of the untwisted carbon nanotube wire can bearbitrarily set as desired. A diameter of the untwisted carbon nanotubewire can be in a range from about 0.5 nm to about 100 nm. The untwistedcarbon nanotube wire is formed by treating the carbon nanotube film withan organic solvent. Specifically, the carbon nanotube film is treated byapplying the organic solvent to the carbon nanotube film to soak theentire surface of the carbon nanotube film. After being soaked by theorganic solvent, the adjacent paralleled carbon nanotubes in the carbonnanotube film will bundle together, due to the surface tension of theorganic solvent as the organic solvent volatilizes, and thus, the carbonnanotube film will shrink into an untwisted carbon nanotube wire.

The twisted carbon nanotube wire is formed by twisting a carbon nanotubefilm by using a mechanical force to turn the two ends of the carbonnanotube film in opposite directions. Referring to FIG. 6, the twistedcarbon nanotube wire includes a plurality of carbon nanotubes orientedaround an axial direction of the twisted carbon nanotube wire. Thecarbon nanotubes are aligned around the axis of the carbon nanotubetwisted wire like a helix. More specifically, the twisted carbonnanotube wire includes a plurality of successive carbon nanotubesegments joined end to end by van der Waals attractive forcetherebetween. Each carbon nanotube segment includes a plurality ofcarbon nanotubes substantially parallel to each other, and combined byvan der Waals attractive force therebetween. The carbon nanotubesegments can vary in width, thickness, uniformity and shape. The lengthof the carbon nanotube wire can be arbitrarily set as desired. Adiameter of the twisted carbon nanotube wire is in an range from about0.5 nm to about 100 nm.

Furthermore, the twisted carbon nanotube wire can be treated with avolatile organic solvent. After being soaked by the organic solvent, theadjacent paralleled carbon nanotubes in the twisted carbon nanotube wirewill bundle together, due to the surface tension of the organic solventas the organic solvent volatilizes. The specific surface area of thetwisted carbon nanotube wire will decrease, and the density and strengthof the twisted carbon nanotube wire will increase. Examples of thecarbon nanotube wire are taught by U.S. Pat. No. 7,045,108 to Jiang etal., and US 20100173037 A1 to Jiang et al.

As discussed above, the carbon nanotube layer 104 can be used as a maskfor growing the first semiconductor layer 106. The term ‘mask’ forgrowing the semiconductor layer 106 means that the carbon nanotube layer104 can be used to shelter a part of the epitaxial growth surface 122and expose the other part of the epitaxial growth surface 122. Thus, thesemiconductor layer 106 can grow from the exposed epitaxial growthsurface 122. The carbon nanotube layer 104 can form a pattern mask onthe epitaxial growth surface 122 because the carbon nanotube layer 104defines a plurality of first apertures 105. Compared to lithography oretching, the method of forming a carbon nanotube layer 104 as mask issimple, low in cost, and will not pollute the substrate 102.

In step S30, the first semiconductor layer 106, the active layer 108 andthe second semiconductor layer 110 are grown in sequence by a molecularbeam epitaxy, chemical beam epitaxy, vacuum epitaxy, low temperatureepitaxy, selective epitaxy, liquid phase deposition epitaxy, metalorganic vapor phase epitaxy, ultra-high vacuum chemical vapordeposition, hydride vapor phase epitaxy, and metal organic chemicalvapor deposition.

A thickness of the first semiconductor layer 106 can be selectedaccording to need. The thickness of the first semiconductor layer 106can be in a range from about 1 nm to about 15 nm. In one embodiment, thethickness of the first semiconductor layer 106 is about 2 nm. The firstsemiconductor layer 106 includes an intrinsic semiconductor layer 101and a doped semiconductor layer 111. The doped semiconductor layer 111can be an N-type semiconductor layer or a P-type semiconductor layer.The N-type semiconductor layer provides electrons, and the P-typesemiconductor layer provides cavities. The N-type semiconductor layercan be made of N-type gallium nitride, N-type gallium arsenide, orN-type copper phosphate. The P-type semiconductor layer can be made ofP-type gallium nitride, P-type gallium arsenide, or P-type copperphosphate. In one embodiment, the doped semiconductor layer 111 is aSi-doped N-type gallium nitride semiconductor layer.

The active layer 108 is a photon exciting layer and can be one of asingle quantum well layer or multilayer quantum well films. The activelayer 108 can be made of gallium indium nitride (GaInN), aluminum indiumgallium nitride (AlGaInN), gallium arsenide (GaSn), aluminum galliumarsenide (AlGaSn), gallium indium phosphide (GaInP), or aluminum galliumarsenide (GaInSn). The active layer 108, in which the cavities thereinare filled by the electrons, can have a thickness of about 0.01 nm toabout 0.6 nm. In one embodiment, the active layer 108 has a thickness ofabout 0.3 nm and includes a layer of InGaN/GaN.

The second semiconductor layer 110 can be an N-type semiconductor layeror a P-type semiconductor layer. The type of the second semiconductorlayer 110 is different from the type of the first semiconductor layer106. If the first semiconductor layer 106 is an N-type semiconductor,the second semiconductor layer 110 is a P-type semiconductor, and viceversa. A thickness of the second semiconductor layer 110 is in a rangefrom about 0.1 nm to about 3 nm. A surface of the second semiconductorlayer 110 away from the substrate 102 can act as a light-emitting face.In one embodiment, the second semiconductor layer 110 can be an Mg-dopedP-type gallium nitride semiconductor layer and a thickness of the secondsemiconductor layer 110 is about 0.3 nm.

In one embodiment, the first semiconductor layer 106 is prepared bymetal organic chemical vapor deposition method. The carrier gas includesH₂, N₂ or a mixture thereof. The trimethyl gallium is used as Ga source,the silane is used as the silicon source, and ammonia is used as anitrogen source gas. The method for making the first semiconductor layer106 includes the following steps:

S31, putting the substrate 102 with the carbon nanotube layer 104thereon into a reaction chamber, flowing a carrier gas into the reactionchamber, and heating the reaction chamber to about 1100° C. to about1200° C. for about 200 sec to about 1000 sec;

S32, growing a low-temperature GaN layer by cooling the reaction chamberto about 500° C. to about 650° C. and flowing trimethyl gallium andammonia gas into the reaction chamber;

S33, stopping the flow of the trimethyl gallium, heating and maintainingthe temperature of the reaction chamber at about 1100° C. to about 1200°C., and maintaining the temperature of the reaction chamber for about 30seconds to about 300 seconds.

S34, maintaining the temperature of the reaction chamber in a range fromabout 1000° C. to about 1100° C. and the pressure in the reactionchamber at about 100 torr to about 300 torr; and

S35, growing the doped semiconductor layers 111 by maintaining thetemperature of the reaction chamber at about 1000° C. to about 1100° C.,and flowing silane into the reaction chamber.

In step S31, the carrier gas includes H₂, N₂ or a mixture thereof. Thesubstrate 102 can be sapphire.

In step S32, the trimethyl gallium can be substituted by the triethylgallium. The low-temperature GaN layer is used as a buffer layer. Athickness of the low-temperature GaN layer is in a range from about 10nm to about 50 nm. The low-temperature GaN layer can reduce the latticemismatch between the first semiconductor layer 106 and the sapphiresubstrate 102. Therefore, the dislocation density of the firstsemiconductor layer 106 will be low. The material of the buffer layercan also be aluminium nitride.

In step S34, a high-temperature GaN layer is obtained. A thickness ofthe high-temperature GaN layer is in a range from about 200 nm to about10 nm. The high-temperature GaN layer is used as an intrinsicsemiconductor layer 101. The buffer layer, the intrinsic semiconductorlayer 101 and the doped semiconductor layers 111 together are defined asthe first semiconductor layer 106.

The growth process of the first semiconductor layer 106 can be dividedinto the following stages:

nucleating on the epitaxial growth surface 122 and growing a pluralityof epitaxial crystal grains along the direction substantiallyperpendicular to the epitaxial growth surface 122.

forming a continuous epitaxial film by growing the epitaxial crystalgrains along the direction substantially parallel to the epitaxialgrowth surface 122.

forming a high-grade epitaxial film by growing the epitaxial film alonga direction substantially perpendicular to the epitaxial growth surface122.

In the first stage, the epitaxial crystal grains grow from the exposedpart of the epitaxial growth surface 122 and through the aperture 105.The growth of the epitaxial crystal grains along the directionsubstantially perpendicular to the epitaxial growth surface 122 iscalled vertical epitaxial growth.

In the second stage, the epitaxial crystal grains are joined together toform an integral structure to cover the carbon nanotube layer 104. Theepitaxial crystal grains grow to enclose the carbon nanotubes of thecarbon nanotube layer 104 and form a plurality of grooves on the bottomsurface of the first semiconductor layer 106. The grooves are covered bythe substrate 102 to form a plurality of channels 103. The inner wall ofthe channels 103 can be in contact with the carbon nanotubes or spacedfrom the carbon nanotubes, which depends on whether the material of theepitaxial film and the carbon nanotubes have mutual infiltration. Thus,the epitaxial film defines a patterned depression on the surfaceoriented to the epitaxial growth surface 122. The patterned depressioncorresponds to the patterned carbon nanotube layer 104. If the carbonnanotube layer 104 includes a layer of parallel and spaced carbonnanotube wires, the patterned depression is a plurality of parallel andspaced grooves. If the first carbon nanotube layer 104 includes aplurality of carbon nanotube wires crossed or weaved together to form acarbon nanotube net, the patterned depression is a grooves networkincluding a plurality of cross-set grooves. The cross section of thechannel 103 can be geometrically shaped. The biggest diameter of thechannel 103 is in a range from about 20 nm to about 200 nm. In oneembodiment, the biggest diameter of the channel 103 is in a range fromabout 50 nm to about 100 nm. The carbon nanotube layer 104 can preventthe lattice dislocation between the epitaxial crystal grains and thesubstrate 102 from growing. The growth of epitaxial crystal grains alongthe direction substantially parallel to the epitaxial growth surface 122is called lateral epitaxial growth.

In the third stage: the first semiconductor layer 106 is obtained. Theepitaxial crystal grains, the epitaxial film and the high-gradeepitaxial film constitute the first semiconductor layer 106. Because thecarbon nanotube layer 104 can prevent the lattice dislocation betweenthe epitaxial crystal grains and the substrate 102 from growing in step(302), the first semiconductor layer 106 has less defects therein.

A method for growing the active layer 108 is similar to the method forgrowing the first semiconductor layer 106. The active layer 108 is grownafter growing the first semiconductor layer 106. In one embodiment, themethod for growing the active layer 108 includes the following steps:

step a1, stopping the flow of the silane into the reaction chamber afterthe step S35 of growing the first semiconductor layer 106, heating thereaction chamber to a temperature of about 700° C. to about 900° C., andmaintaining the pressure of the reaction chamber at about 50 torr toabout 500 torr.

step a2, growing InGaN/GaN multi-quantum well layer to form the activelayer 108 by flowing trimethyl indium into the reaction chamber.

A method for growing the second semiconductor layer 110 is similar tothe method for growing the first semiconductor layer 106. The secondsemiconductor layer 110 is grown after growing the active layer 108. Inone embodiment, the method for growing the second semiconductor layer110 includes the following steps:

step b1, stopping the flow of the trimethyl indium into the reactionchamber after the step a2 of growing the active layer 108, heating thereaction chamber to a temperature of about 1000° C. to about 1100° C.,and maintaining pressure of the reaction chamber at about 76 torr toabout 200 torr; and

step b2, growing Mg-doped P-type GaN layer to form the secondsemiconductor layer 110 by flowing ferrocene magnesium into the reactionchamber.

After the second semiconductor layer 110 is obtained, a cross sectionbetween the substrate 102 and the first semiconductor layer 106 isobserved and tested by a TEM. Referring to FIG. 7, a light-coloredportion in the TEM picture is the sapphire substrate 102, and adark-colored portion in the TEM picture is the first semiconductor layer106. The first semiconductor layer 106 only grows from a portion surfaceof the epitaxial growth surface 122 exposed by the aperture 105 of thecarbon nanotube layer 104. A number of channels are defined between thesubstrate 102 and the first semiconductor layer 106. The carbonnanotubes are located in the channels and spaced from the firstsemiconductor layer 106.

Furthermore, a highly doped semiconductor electrode contact layer can belocated on a top surface of the second semiconductor layer 110. Thehighly doped semiconductor electrode contact layer can be obtained by amethod similar to the method for making the second semiconductor layer110, and the only difference is to change the content of doping elementsin the source gas during the growing progress.

In step S40, the second semiconductor layer 110, and the active layer108 are etched by a reactive ion etching. After the active layer 108 isetched, the first semiconductor layer 106 can also be etched by thereactive ion etching. After the first semiconductor layer 106 is etched,the carbon nanotube layer 104 is covered by the semiconductor layer 106.The substrate 102, the carbon nanotube layer 104, the firstsemiconductor layer 106, the active layer 108 and the secondsemiconductor layer 110 constitute an LED chip.

In one embodiment, the active layer 108 is made of InGaN/GaN layer andthe second semiconductor layer 110 is made of P-type GaN layer. Thesecond semiconductor layer 110 and the active layer 108 can be etched byplacing the LED chip into an inductively coupled plasma device andadding a mixture of silicon tetrachloride and chlorine into theinductively coupled plasma device. In one embodiment, the power of theinductively coupled plasma device is about 50 W, the speed of thechlorine is about 26 sccm, and the speed of the silicon tetrachloride isabout 4 sccm. The partial pressure of the silicon tetrachloride andchlorine is about 2 Pa. The etched thickness of the second semiconductorlayer 110 is about 0.3 nm. The etched thickness of the active layer 108is about 0.3 nm.

In step S50, the first electrode 114 is located on the exposed surfaceof the first semiconductor layer 106, and the second electrode 112 islocated on a top surface of the second semiconductor layer 110. Thefirst electrode 114 may be a P-type or an N-type electrode and is thesame type as the first semiconductor layer 106. The second electrode 112may be a P-type or an N-type electrode and is the same type as thesecond semiconductor layer 110.

A thickness of the first electrode 114 can range from about 0.01 nm toabout 2 nm. A thickness of the second electrode 112 can range from about0.01 nm to about 2 nm. The first electrode 114 can be made of titanium,aluminum, nickel, gold, or a combination thereof. In one embodiment, thefirst electrode 114 is an N-type electrode and includes a nickel layerand a gold layer. A thickness of the nickel layer is about 150 angstroms(Å). A thickness of the gold layer is about 1000 Å. In one embodiment,the second electrode 112 is a P-type electrode and includes a titaniumlayer and a gold layer. A thickness of the titanium layer is about 150Å. A thickness of the gold layer is about 2000 Å. After the step S50, anLED preform is obtained.

In step S60, the carbon nanotube layer 104 can be removed by a plasmaetching method, a laser heating method, or a furnace heating method. Thecarbon nanotubes in the carbon nanotube layer 104 will be oxidized andremoved.

In one embodiment, the plasma etching method for removing the carbonnanotube layer 104 includes the following steps:

S612, placing the LED preform into a vacuum chamber; and

S614, flowing reactive gases into the vacuum chamber to obtain plasmasof the reactive gases to etch the carbon nanotube layer 104.

In step S612, the vacuum chamber can be a vacuum chamber of a reactiveion etching machine.

The step S614 can include the following substeps:

S6142, pumping the vacuum chamber of the reactive ion etching machineinto a vacuum;

S6144, flowing the reactive gases into the vacuum chamber; and

S6146, producing the plasmas of the reactive gases by glow dischargemethod.

In step S6144, the reactive gases can be oxygen gas, hydrogen, ortetrafluoromethane.

In step S6146, the plasmas include charged ions and electrons. Theplasmas can be oxygen plasmas, hydrogen plasmas or tetrafluoromethaneplasmas. The plasmas can be selected according to need. In oneembodiment, the reactive gas is oxygen gas and the plasmas are oxygenplasmas. Since the plasmas have a better mobility and the carbonnanotubes in the channels 103 are spaced apart from the firstsemiconductor 106, the plasmas can flow into the channels 103 of the LEDpreform by controlling the flow time and pressure of the plasmas. Theplasmas collide with the carbon nanotubes in the channels 103 tophysically etch and remove the carbon nanotubes. The plasmas and thecarbon nanotubes may have an oxidation reaction, and carbon dioxide orother volatile reaction products will be produced. The plasmas and thecarbon nanotubes need sufficient reaction time to remove the carbonnanotubes. A power of the glow discharge is in a range from about 20watts (W) to about 300 W. In one embodiment, the power of the glowdischarge is about 150 W. A flow rate of the reaction gas is in a rangefrom about 10 ml/sccm to about 100 ml/sccm. In one embodiment, the flowrate of the reaction gas is about 10 ml/sccm. A pressure of the vacuumchamber is in a range from about 1 Pa to about 100 Pa. A reaction timeis in a range from about 10 sec to about 1 hour. In one embodiment, thereaction time is in a range from about 15 sec to about 15 minutes.

In one embodiment, the laser heating method for removing the carbonnanotube layer 104 includes the following steps:

S622, providing a laser generator which can generate a laser beam toirradiate the bottom surface of the substrate 102 of the LED preform;

S624, scanning the bottom surface of the substrate 102 of the LEDpreform by making a relative movement between the laser and thesubstrate 102 in an environment containing oxygen gas.

In step S622, the laser is perpendicular to the exposed bottom surfaceof the substrate 102 away from the carbon nanotube layer 104. The lasergenerator can be a solid laser generator, liquid laser generator, gaslaser generator, and semiconductor laser generator. A power density ofthe laser is larger than 0.053×10¹² W/m². As the laser irradiates thesurface of the substrate 102, a laser beam produced by the lasergenerator is focused on the surface of substrate 102 and forms a laserirradiating area, e.g., a circle area, on the substrate 102 in which adiameter of the laser irradiating area can range from about 1 mm toabout 5 mm. As a plurality of laser generators works together to producea strip laser irradiating area, a width of the strip laser irradiatingarea is in a range from about 1 mm to about 5 mm. An irradiating time ofthe laser is shorter than 1.8 sec. In one embodiment, the lasergenerator is a carbon dioxide laser generator, a power of the carbondioxide laser generator is about 30 watts, a wavelength of the laser isabout 10.6 nm, and a diameter of the laser irradiating area is about 3mm.

The first semiconductor layer 106 should be stable under the irradiationof the laser. If the first semiconductor layer 106 includes thelow-temperature GaN layer, the wavelength of the laser cannot be about248 nm because the low-temperature GaN layer will decompose under theirradiation of the laser at a wavelength of about 248 nm.

The step S624 can be executed by two methods. The first method isexecuted by fixing the LED preform and moving a mobile laser generatorto irradiate the LED preform. The second method is executed by fixingthe laser generator and moving a mobile LED preform to make the LEDpreform be irradiated by the laser generator. A moving direction of thelaser beam relative to the substrate can be substantially perpendicularor parallel to the axial direction of the carbon nanotubes in the carbonnanotube layer 104.

If the substrate 102 is opaque and the substrate 102 is irradiated bythe laser, the substrate 102 is heated by the laser. The heat isconducted to the carbon nanotube layer 104 via the substrate 102 to heatthe carbon nanotube layer 104. The oxygen gas can flow into the channels103 easily because the carbon nanotubes are spaced from the firstsemiconductor layer 106, thus, the carbon nanotubes absorb the heat andreact with the oxygen gas, and the carbon nanotube layer 104 will beoxidized into carbon dioxide gas and removed.

If the substrate 102 is transparent, the laser can pass through thesubstrate 102 and irradiate the carbon nanotubes directly. The carbonnanotubes can absorb the energy of the laser and be heated by the laser,so that the carbon nanotubes can react with the oxygen gas and beremoved. The reaction time can be controlled by adjusting the relativemoving speed between the laser generator and the LED preform. If thepower density and the wavelength of the laser are fixed, the slower themoving speed of the laser generator or the LED preform, and the longerthe irradiation time of the carbon nanotubes. The longer the irradiationtime of the carbon nanotubes, the more energy the carbon nanotubesabsorb. The more energy that the carbon nanotubes absorb, the easier thecarbon nanotubes oxidize. In one embodiment, the relative moving speedof the laser generator and the carbon nanotube layer 104 is less than 10mm/sec.

In one embodiment, the furnace heating method for removing the carbonnanotube layer 104 include the following steps:

S632: providing a heating furnace having a chamber and placing the LEDpreform into the chamber;

S634: flowing oxygen gas into the chamber and heating the LED preform.

In step S632, the structure of the furnace is not limited. In oneembodiment, the furnace is a resistance furnace.

In step S634, the carbon nanotubes in the carbon nanotube layer 104 canabsorb the heat in the heating furnace and be oxidized by the oxygengas. The heating temperature of the heating furnace is higher than 600°C. to ensure that the carbon nanotubes can absorb enough heat and beoxidized. In one embodiment, the temperature of the heating furnace isheated to about 650° C. to remove the carbon nanotubes. After the carbonnanotubes are removed, the LED 10 is obtained.

In each of the plasma etching method, the laser heating method and thefurnace heating method is simple and creates no pollution.

The method for making LED 10 has the following merits that:

First, the method is simple and low-cost because the carbon nanotubelayer 104 acts as a mask to form a number of channels 103 between thefirst semiconductor layer 106 and the substrate 102 without strippingthe substrate 102. Second, the carbon nanotube layer 104 is afree-standing structure, therefore, the carbon nanotube layer 104 can bedirectly located on the substrate 102.

Referring to FIG. 8, one embodiment of an LED 10 is provided. The LED 10includes a substrate 102, a intrinsic semiconductor layer 101, a dopedsemiconductor layer 111, an active layer 108, a second semiconductorlayer 110, a first electrode 114 and a second electrode 112. Theintrinsic semiconductor layer 101, the doped semiconductor layer 111,the active layer 108 and the second semiconductor layer 110 are locatedon the substrate 102 in that order. The intrinsic semiconductor layer101 and the doped semiconductor layer 111 are a first semiconductorlayer 106. The first semiconductor layer 106 is located on the substrate102. The first electrode 114 is electrically connected with the firstsemiconductor layer 106. The second electrode 112 is electricallyconnected with the second semiconductor layer 110. A number of groovesare defined on a bottom surface of the first semiconductor layer 106contacting with the substrate 102. The grooves are covered by thesubstrate 102 to form a number of channels 103. The channels 103 arebetween the first semiconductor layer 106 and substrate 102. The firstsemiconductor layer 106 contacts a portion surface of the substrate 102to define a number of channels 103.

The cross section of the channels 103 can be geometrically shaped. Adiameter of the channels 103 can be in a range from about 20 nm to about200 nm. In one embodiment, the diameter of the channels 103 can be in arange from about 50 nm to about 100 nm. If the carbon nanotube layer 104includes a number of carbon nanotubes and axial directions of the carbonnanotubes is oriented along one direction, the grooves are a pluralityof strip grooves paralleled to and spaced from each other, and thechannels 103 are a plurality of strip channels 103 paralleled to andspaced from each other. If the carbon nanotube layer 104 includes anumber of carbon nanotube wires parallel to and spaced from each other,the channels 103 are a plurality of strip channels paralleled to andspaced from each other. If the carbon nanotube layer 104 is composed ofcarbon nanotube wires crossed with each other or woven together to forma network structure or the carbon nanotube layer 104 is composed of anumber of cross-stacked carbon nanotube film, the grooves are crossedwith each other or woven together form a groove network. If the carbonnanotube layer 104 is composed of a number of cross-stacked carbonnanotube films, an angle defined between the carbon nanotubes in twoadjacent carbon nanotube films is greater than 0 degrees and less than90 degrees. The grooves are composed of a plurality first paralleledgrooves crossed with a plurality of second paralleled groovessubstantially perpendicular to the first grooves. The grooves arecrossed with each other to form a groove network. The grooves are covedby the substrate 102 to form a number of channels 103 between the firstsemiconductor layer 106 and the substrate 102. The channels 103 areinterconnected. The channels 103 form a channel network. The channels103 can scatter light from the active layer 108 and improve theextraction efficiency of the LED 10.

Referring to FIG. 9, a method for making an LED 20 of one embodimentincludes the following steps of:

S100: providing a substrate 202 having an epitaxial growth surface 222;

S200: placing a first carbon nanotube layer 204 on the epitaxial growthsurface 222;

S300: growing a first semiconductor layer 206, an active layer 208 and asecond semiconductor layer 210 on the epitaxial growth surface 222successively;

S400: placing a second carbon nanotube layer 207 on the secondsemiconductor layer 210;

S500: growing a third semiconductor layer 209 on a surface of the secondsemiconductor layer 210 away from the substrate 202, wherein the thirdsemiconductor layer 209 includes a number of protrusions spaced fromeach other;

S600: removing the first carbon nanotube layer 204 and the second carbonnanotube layer 207;

S700: exposing a surface of the first semiconductor layer 206 by etchingthe third semiconductor layer 209, the second semiconductor layer 210and the active layer 208; and

S800: preparing a first electrode 214 on the exposed surface of thefirst semiconductor layer 206, and preparing a second electrode 212 onthe second semiconductor layer 210.

The method for making the LED 20 in FIG. 9 is similar to the method formaking the LED 10 in FIG. 1. The steps S100, S200, and S300 of themethod for making the LED 20 are identical to the steps S10, S20, andS30 of the method for making the LED 10, except that a thirdsemiconductor layer 209 is formed on the second semiconductor layer 210.

In step S400, the second carbon nanotube layer 207 is a mask during thegrowth process of the third semiconductor layer 209. The second carbonnanotube layer 207 is identical with the first carbon nanotube layer 104in FIG. 1. The third semiconductor layer 209 grows from the secondsemiconductor layer 210 corresponding to the apertures 205 of the secondcarbon nanotube layer 207. After step S400, an LED chip is obtained.

In step S500, a number of third semiconductor particles nucleate andepitaxially grow on a top surface of the second semiconductor layer 210away from the substrate 202. The third semiconductor particles grow fromthe apertures 205 of the second carbon nanotube layer 207 along adirection substantially perpendicular to the top surface of the secondsemiconductor layer 210. The third semiconductor particles are a numberof protrusions 215 spaced by the second carbon nanotube layer 207. Theprotrusions 215 constitute the third semiconductor layer 209. The thirdsemiconductor layer 209 is discontinuous and spaced from each other toform a number of openings 213 defined by the third semiconductor layer209. The carbon nanotubes in the second carbon nanotube layer 207 arelocated in the openings 213. The third semiconductor layer 209 can be anumber of substantially paralleled strip protrusions 215 or anprotrusions array. A thickness of the third semiconductor layer 209 canbe controlled by adjusting the growth time of the third semiconductorparticles. The material of the third semiconductor layer 209 can begallium nitride, gallium arsenide, and copper phosphide. The material ofthe third semiconductor layer 209 is the same as the secondsemiconductor layer 210. In one embodiment, the material of the thirdsemiconductor layer 209 is Mg-doped GaN.

In one embodiment, the step S500 includes the following substeps:

placing the LED chip obtained after the step S400 into a chamber,heating the chamber to a temperature between about 1100° C. to about1200° C., flowing a mixture gas of H₂ and N₂ as carrier gas, and heatingthe chamber for about 200 sec to about 1000 sec to purify the LED chip.

maintaining the temperature of the chamber at about 1000° C. to about1100° C. and the pressure of the chamber at about 76 torr to about 200torr, and flowing trimethyl gallium, and ferrocene magnesium to growMg-doped P-type GaN layer. The Mg-doped P-type GaN layer constitutes thethird semiconductor layer 209. A thickness of the third semiconductorlayer 209 is in a range from about 10 nm to about 50 nm.

In step S600, the first carbon nanotube layer 204 and the second carbonnanotube layer 207 are removed by the same method as the step S60 of themethod for making the LED 10 of FIG. 1. The first carbon nanotube layer204 and the second carbon nanotube layer 207 can be removed by methodssuch as plasma etching, laser heating, or furnace heating. After thefirst carbon nanotube layer 104 is removed, a number of nano-scalegrooves are defined on a bottom surface of the first semiconductor layer206 oriented to the substrate 202. The grooves are covered by thesubstrate 202 to form a number of channels 203. After the second carbonnanotube layer 207 is removed, a number of protrusions 215 are obtainedand constitute the third semiconductor layer 209. The thirdsemiconductor layer 209 is a nano-scale microstructure formed on a topsurface of the second semiconductor layer 210.

In one embodiment, in step S600, the second carbon nanotube layer 207 isnot removed and the protrusions 215 of the third semiconductor layer 209can be spaced by the second carbon nanotube layer 207. The second carbonnanotube layer 207 is transparent and can be electrically connected tothe second electrode 212.

The step S700 is similar to the step S40 of the method for making theLED 10 shown in FIG. 1. The difference is that the third semiconductorlayer 209 is etched with the second semiconductor layer 210 and theactive layer 208 together. The third semiconductor layer 209 can beetched by reactive ion etching method.

The step S800 is similar to the step S50 of the method for making theLED 10 shown in FIG. 1. The difference is that the second electrode 212covers a portion of the third semiconductor layer 209 located below thesecond electrode 212. The third semiconductor layer 209 includes anumber of protrusions. The protrusions are spaced from each other todefine a number of openings 213 in the third semiconductor layer 209.The second electrode 212 penetrates the openings 213 and contacts withthe second semiconductor layer 210. After the second electrode 212 isobtained, a portion of the third semiconductor layer 209 is locatedbetween the second electrode 212 and the second semiconductor layer 210.

The method for making the LED 20 includes the following merits. First,the second carbon nanotube layer 207 is a mask to grow the thirdsemiconductor layer 209, therefore the method is simple and low in costcompared to traditional nano-imprint technology. Second, the secondcarbon nanotube layer 207 is a free-standing structure and can belocated on the top surface of the second semiconductor layer 210,therefore, the method is simple and is adapted to large-scale industrialmanufacturing.

Referring to FIG. 10, an LED 20 of one embodiment is illustrated. TheLED 20 includes a substrate 202, a first semiconductor layer 206, anactive layer 208, a second semiconductor layer 210, a thirdsemiconductor layer 209, and a first electrode 214 and a secondelectrode 212. The first semiconductor layer 206 includes a buffer layer(not shown in FIG. 10), an intrinsic semiconductor layer 201 and a dopedsemiconductor layer 211. The buffer layer, the intrinsic semiconductorlayer 201, the doped semiconductor layer 211, the active layer 208, andthe second semiconductor layer 210 are located on one side of thesubstrate 202 in that order. The first semiconductor layer 206 islocated oriented to the substrate 202. The first electrode 214 iselectrically connected to the first semiconductor layer 206. The secondelectrode 212 is electrically connected to the second semiconductorlayer 210. A number of nano-scale grooves are defined on a bottomsurface of the first semiconductor layer 206 contacting the substrate202. The grooves are covered by the substrate 202 to form a number ofchannels 203 between the substrate 202 and the first semiconductor layer210. A third semiconductor layer 209 is located on a top surface of thesecond semiconductor layer 210 away from the substrate 202.

The structure of the LED 20 is similar to the structure of the LED 10.The difference is that the third semiconductor layer 209 is located onthe top surface of the second semiconductor layer 210 away from thesubstrate 202. The third semiconductor layer 209 includes a number ofprotrusions spaced from each other. The cross section of the protrusionscan have irregular geometric shapes. In one embodiment, the thirdsemiconductor layer 209 includes a number of strip protrusions parallelto and spaced from each other. The extending directions of the stripprotrusions can the same as or different from the extending direction ofthe strip channels 203. In one embodiment, the extending direction ofthe strip protrusions is substantially perpendicular to the extendingdirection of the strip channels 203. The strip protrusions are spacedfrom each other by the strip openings 213. A width of the stripprotrusions is in a range from about 10 nm to about 10 nm. In oneembodiment, the width of the strip protrusions is in a range from about50 nm to about 100 nm. A width of the strip openings 213 is in a rangefrom about 20 nm to about 200 nm. The third semiconductor layer 209includes a number of dot-like protrusions in one embodiment. Thedot-like protrusions are arranged in an array. The protrusions arespaced from each other to form a number of openings. The secondelectrode 212 penetrates the openings and contacts the secondsemiconductor layer 210.

The light extraction rate of the LED 20 is improved because the thirdsemiconductor layer 209 has a number of nano-scale protrusions 215.Therefore, the directions of the large-angle lights are changed and exitfrom the LED 20 if the large-angle lights produced in the active layer208 transmit to the third semiconductor layer 209.

It is to be understood that the above-described embodiments are intendedto illustrate rather than limit the disclosure. Any elements describedin accordance with any embodiments is understood that they can be usedin addition or substituted in other embodiments. Embodiments can also beused together. Variations may be made to the embodiments withoutdeparting from the spirit of the disclosure. The above-describedembodiments illustrate the scope of the disclosure but do not restrictthe scope of the disclosure.

Depending on the embodiment, certain of the steps of methods describedmay be removed, others may be added, and the sequence of steps may bealtered. It is also to be understood that the description and the claimsdrawn to a method may include some indication in reference to certainsteps. However, the indication used is only to be viewed foridentification purposes and not as a suggestion as to an order for thesteps.

What is claimed is:
 1. A method for making a light emitting diode, themethod comprising: providing a substrate having an epitaxial growthsurface; placing a carbon nanotube layer on the epitaxial growthsurface; growing a first semiconductor layer, an active layer and asecond semiconductor layer on the epitaxial growth surface; exposing aportion of the first semiconductor layer by etching a portion of thesecond semiconductor layer and the active layer; forming a lightemitting diode preform by preparing a first electrode on the firstsemiconductor layer and a second electrode on the second semiconductorlayer; and removing the carbon nanotube layer.
 2. The method of claim 1,wherein the carbon nanotube layer is a free-standing structure andlocated on the epitaxial growth surface directly.
 3. The method of claim1, wherein the carbon nanotube layer defines a plurality of apertures.4. The method of claim 3, wherein the apertures extend through thecarbon nanotube layer along a direction that is parallel to the carbonnanotube layer.
 5. The method of claim 3, wherein a size of each of theapertures is in a range from about 10 nanometers to about 300micrometers.
 6. The method of claim 3, wherein during the step ofgrowing the first semiconductor layer, the first semiconductor layerepitaxially grows from the epitaxial growth surface and penetrates theapertures of the carbon nanotube layer.
 7. The method of claim 6,wherein the growth process of the first semiconductor layer comprises:nucleating on the epitaxial growth surface and growing a plurality ofepitaxial crystal grains along a direction substantially perpendicularto the epitaxial growth surface; forming a continuous epitaxial film bygrowing the epitaxial crystal grains along a direction substantiallyparallel to the epitaxial growth surface; and growing the epitaxial filmalong the direction substantially perpendicular to the epitaxial growthsurface to form the first semiconductor layer.
 8. The method of claim 1,wherein the carbon nanotube layer comprises one carbon nanotube film ora plurality of wires substantially parallel to and spaced from eachother.
 9. The method of claim 8, wherein a plurality of strip grooves isdefined by the first semiconductor layer, and the strip grooves aresubstantially parallel to and spaced from each other.
 10. The method ofclaim 9, wherein the grooves are covered by the substrate to form aplurality of strip channels between the first semiconductor layer andsubstrate, the strip channels are substantially parallel to and spacedfrom each other.
 11. The method of claim 10, wherein the carbon nanotubelayer is located in the strip channels.
 12. The method of claim 1,wherein the carbon nanotube layer comprises a plurality of cross-stackedcarbon nanotube films or a plurality of carbon nanotube wires crossedwith each other or woven together to form a network.
 13. The method ofclaim 12, wherein a plurality of the strip grooves is defined by thefirst semiconductor layer, and the strip grooves are crossed with eachother to form a groove network.
 14. The method of claim 13, wherein thegrooves are covered by the substrate to form a plurality of channelscrossed with each other to form a channel network.
 15. The method ofclaim 1, wherein the carbon nanotube layer is removed by a plasmaetching method comprising: placing the light emitting diode preform intoa vacuum chamber of the reactive ion etching machine; and obtainingplasmas of the reactive gases to etch the carbon nanotube layer byflowing reactive gases into the vacuum chamber.
 16. The method of claim15, wherein the plasmas are oxygen plasmas, hydrogen plasmas, ortetrafluoromethane plasmas.
 17. The method of claim 1, wherein thecarbon nanotube layer is removed by a laser heating method comprising:irradiating a surface of the substrate away from the carbon nanotubelayer of the light emitting diode preform using a laser; scanning thesurface of the substrate of the light emitting diode preform by relativemovement between the laser and the substrate in an environmentcontaining oxygen gas.
 18. The method of claim 1, wherein the carbonnanotube layer is removed by a furnace heating method comprising:providing a heating furnace having a chamber and placing the lightemitting diode preform into the chamber; and flowing oxygen gas into thechamber and heating the light emitting diode preform.
 19. The method ofclaim 18, wherein a heating temperature of the heating furnace is higherthan 600° C.
 20. A method for making a light emitting diode, the methodcomprising: providing a substrate having an epitaxial growth surface;providing a first carbon nanotube layer on the epitaxial growth surface,wherein the first carbon nanotube layer defines a plurality of firstapertures; growing a first semiconductor layer, an active layer, and asecond semiconductor layer on the epitaxial growth surface successively;locating a second carbon nanotube layer on the second semiconductorlayer, wherein the second carbon nanotube layer defines a plurality ofsecond apertures; growing a third semiconductor layer on a surface ofthe second semiconductor layer away from the substrate, wherein thethird semiconductor layer comprises a plurality of protrusions spacedfrom each other; removing the first carbon nanotube layer; exposing asurface of the first semiconductor layer by etching the thirdsemiconductor layer, the second semiconductor layer, and the activelayer; preparing a first electrode on the exposed surface of the firstsemiconductor layer and preparing a second electrode on the secondsemiconductor layer.